A Unique Approach to Teaching Spectrophotometric Procedures The teaching of spectrophotometric procedures usually requires several hours of preparations in careful weighing5 and sample labelings. Substituting commercial food and clothing dyes for the metal complexes required reduces preparation time, tedious record keeping, and allows a greater variation of unknown samples to be issued. In cases of acute shortages of available laboratory reagents this approach offers a n alternative route to teaching spectrophotometric teehniaue. Tne primary purpose uf thm experiment was to design a practical and ecnnumical method of intmduring the student to the operatron of a spertrophotometer and the procedures necessary for determining the nh-orptron curve of a complex, detprmminfi rheoptrmum wavelength. and devcluping accuratedhtion rechniquei. ~~~
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Experimental Fifteen colors or dyes sold by three different companies were each mixed in individual portions of 500 ml of distilled water in the following amounts: 5 to 8 drops of concentrated food color or about 0.2 g of solid dye to each quantity of water. Various colors were mixed together to obtain different shades of colors. The ratio of mixing those colors is given on the MeCormick food color box. A visible spectrum was obtained for each color using research grade spectrophotometers. Close agreement was obtained between absorbance values found with the Spectranic 20 and more expensive spectrophotometers (0.7% deviation from Cary 17). A computer program' was written to calculate molar absorptivities from values derived from the recorded spectra. Assigned concentrations in ppm and simulated molecular weight values were inserted on the data cards in combination with the actual absorbance values. The advantage of applying various arbitrary concentrations and randomly generated molecular weights is to create a number of distinct unknowns to allow each student a singular problem. Approximately 75 ml of an "unknown" solution was given to each student. Students were instructed to obtain absorbance CUNes on the initial solution using the Spectronic 20 and to construct a Beer's Law plot a t the optimum wavelength by making quantitative dilutions of the solution. The molar absorptivity was then calculated from the concentration and molecular weight given for each solution on a posted computer print-out. Properties of Dyes
h D y e Calm
Manufacturer
Mar. nm
Yellow Yellow Green
MeCormick Saver McCormiek
428 428
Green
Saner
Kelly Green
412
625 400 640 400 640
Rit
" A t absorbance values of appmaimately
Aoomximate .. spectra1 Bandwidth, nma
75 100 120 45 100 70 80 160
A~omximste . spectra1 Bandwith, amn ~
Dye Color Red
Red Fuchsia Pink Orange Blue Blue
Manufacturer McCorrniek Sauer Rit Rit Rit McCorrnick Ssuer
h Max. nm
525 525 523 505 410 62R
640
loo 120
110 130
140 70
70
0.6.
The dyes in the table followed Beer'sLaw and remained stable throughout the experiment. All solutions were rechecked by the instructors t o avoid penalizing the students. This approach accomplished the purposes mentioned above. Alternate experimental objectives could have been directed toward the calculation of concentration or molecular weight. This experiment is much more economical than one utilizing inorganic salts. The evaluation of the dyes and food colors used are shown in the table. The reduction of preparation time is easily seen. Each student had a unique calculation t o perform. The errors usually found in reagent contamination are eliminated and the student can identify his source of experimental error. 'Available by writing Howard P. Williams, Box 273, Southern Station, University of Southern Mississippi, Hattiesburg, Ms. 39401 University of Southern Mississippi Hattiesburg, Mississippi 39401
Roy R. Reeves Myron D. Hutson Howard P. Williams
Volume 52, Number 10, October 1975 / 659
